The present invention relates to a compact zoom lens having a small lens aperture for enlarging an image from a light valve such as, mainly, a DMD (Digital Micromirror Device) for forming an image by changing a reflecting direction of light and projecting the image so enlarged on to a screen or the like and a projector which installs the zoom lens.
In recent years, DMDs have been put into practical use in which fine micromirrors (mirror elements) are arranged on a plane in such a manner as to correspond to matching pixels and the angles of the respective micromirrors so arranged are mechanically controlled by the use of the micro-machine technology so as to form an image. Since the DMD is characterized in that it has a faster responding speed and can obtain a brighter image than the liquid crystal panel that has been widely used in this field and is, hence, suitable for realizing a portable projector unit which is small in size and which can provide an image which is highly bright and of high quality, the DMD has been propagating quickly.
When the DMD is used as a light valve in a projector unit, there are imposed limitations on a projection lens that is used at the same time, the limitations being specific to the DMD.
A first limitation is in relation to the f number of the projection lens, and this limitation is considered as the largest limitation in developing small projector units. In the current DMDs, the angle at which micromirrors tilt to represent their ON and OFF positions in producing an image is +/−12°, and by tilting the micromirrors in that way, reflected lights are switched between valid reflected light (valid light) and invalid reflected light (invalid light). Consequently, in the projector unit which utilizes the DMD as a light valve, it constitutes a condition that the valid light needs to be captured while the invalid light is left not captured, and the f number of the projection lens can be induced from this condition. Namely, f=2.4. In reality, since there exists a demand to take in even more light, a smaller f number is required in many cases while taking into consideration a reduction in contrast within a range which produces no actual damage is caused.
In addition, since the condition like this is established on condition that the position of a pupil of the projection lens which lies on a light valve side thereof is constant, in the event that the pupil position of a zoom lens moves, a loss of light quantity is generated which corresponds to a distance over which the pupil has moved, and therefore, optimizing the pupil position at the wide angle end where brightness generally becomes an issue needs to be taken into consideration.
A second limitation relates to the arrangement of a light source system. Since an image circle of the projection lens is desired to be decreased as much as possible in order to miniaturize the projector unit, the arrangement of an optical system for inputting rays of light for projection into the DMD is limited. In order to allow valid light from the DMD to be inputted into the projection lens, the light source system is to be placed substantially in the same direction as (that is, adjacent to) the projection lens. In addition, a space between a lens element of the projection lens which lies closest to the light valve and the light valve (that is, in general, a back focus) is used by both optical systems such as a projection system and the light source system. Because of this, a long back focal length has to be provided for the projection lens and at the same time, in order to secure a light guiding space from the light source, the lens system on the light valve side needs to be designed small.
When considering this from the viewpoint of optical design of a projection lens, this constitutes a limitation on design of a projection lens in which the position of a pupil on the light valve side has to be located in the vicinity of the rear of the projection lens. On the other hand, in order to increase the performance of the projection lens, a large number of lenses need to be combined, and in the event that the large number of lenses are arranged, the overall length of the projection lens has to have a certain length. Then, in the event that the overall length of the projection lens becomes long, in a lens in which the position of an entrance pupil lies at the rear thereof, a problem is caused that the size of a forward lens is naturally increased, and this contradicts the miniaturization.
In this way, although there are imposed the large limitations on the development thereof, a projector unit which adopts a DMD as a light valve is regarded as advantageous over other methods in miniaturizing the projector unit. Currently, portable compact projector units have been broadly propagated, and among them, data projectors constitute a center of interest because of its handiness when making presentations. In addition, in order to make compact a projector unit itself, it is natural that there exists a strong desire for miniaturization of a projection lens for use in the projector unit. In parallel with this, there also exists a desire for multifunctional projection lenses. To this end, a performance related to image quality which results from the correction of aberrations has to satisfy sufficiently the specification of the DMD used, and moreover, from the viewpoint of convenience, not only can a variable power based on a zooming configuration be available but also a projection lens having a large image circle has been started to be required in order to adopt a so-called shift configuration in which the center of the DMD is caused to deviate from the optical axis of the projection lens, and as a result, a projection lens having a large angle of view at its wide angle end, as well as a projection lens having a large variable power ratio are now being required.
A projection lens which has been developed to such a specification has a tendency that the aperture of a front group lens becomes larger than required in any ways, which comes to affect largely the thickness dimension of the projector unit. However, it is critical to the projector unit which is developed as a portable one that the thickness dimension is decreased, and this factor can be said to be the most critical to the projector unit which is used in many cases in such a way as to be carried together with a note-type personal computer or the like. As an example of a means for solving this problem, for example, the Japanese Unexamined Patent Publication No. 2004-271668 discloses a design method for making compact a projection lens. In this example, however, an effective aperture of a front lens element when a 0.7-inch DMD is used is in the range of 39 mm to 42 mm, and the thickness of the projector unit cannot be decreased to 50 mm or less. When actually carrying the projector unit with this thickness together with a note-type personal computer, the user has to feel that the relevant thickness is still unsatisfactory.
The invention was made in view of the situations described above, and an aspect thereof is to provide a thin projector unit which is compact and bright, which can project an enlarged image on a large screen with high image quality even in a limited space such as a small conference room and which is convenient when carried by realizing a compact zoom lens which is suitable for characteristics of a light valve such as a DMD which forms an image by changing the reflecting direction light, which has a high image forming performance when applied to enlarge an image from the light valve and project the image so enlarged on to a screen or its substitute such as a wall surface and which has a small lens aperture.